Method of coating curved tubing



June 27, 1950 E. MAGER 2,512,866

METHOD OF comma CURVED TUBING Filed Jan. 24, 1946 INVENTOR Eric L. Master BY M ATTORN EY Patented June 27, 1950 Eric L. Mager, Peabody, Mass, assign'or'to Sylvania Electric Products Inc., Salem; Mass; a-

f corporation of Massachusetts Application January v24, 1946, Serial No. 643,047

1o1 aim. (01. 117-435 This invention relates to electric gaseous .dis-

charge devices provided with a coating of lu- A further object of this invention is to provide w a thin, even film of coating material on the interior surface of the glass tubing during the coating process.

Further objects, advantages and features of this invention will be apparent from the following.

description when considered in conjunction with the accompanying drawing in which: Figure 1 is a side elevational view of the apparatus employed in coating curved tubing I Figure 2 is a sectional view taken along the a line 22 of Figure 1; I

Figure 3 is a perspective view of a tubing holder with a curved tube disposed therein and the auxiliary apparatus employed in the coating process connected to the tube;

Figure 4 is a plan view of a curved tube and the auxiliary apparatus employed in the coating process connected thereto, showing the position of the tube and the coating suspension therein at the start of the coating process; and e k 2 v application of a coating of 'luminescent'material tothe' inner walls of a curved tube have proved unsuccessful because no matter how the curved tube is manipulated after its interior surface is flushed with a suspension of luminescent material, it has been found to be impossible to prevent the suspension from drying in. irregular streaks;

Themethod which I have developed to provide a curved tubet with a coating of luminescent materialwhich is smooth in texture and free from streaks "embodies the idea of mounting the curved glass tube in a vertical plane, partially filling it "with a suspension of luminescent material and then rotating the tube slowly so as to" deposit a thin, even film of the suspension on the inner wall of the tube. I

Although'l have found this method of applying the coating to be more satisfactory than the methods described above in connection with the coatingof the conventional elongated tubes, I have found that certain distinct changesmust be made in the nature of the coating suspension 'itself'in order to obtain a satisfactorilyv coated tube. For example, I have found that the solvent Figure 5 is a plan view of the same elements as those illustrated in Figure 4, showing the position of the tube and the coating suspension therein during a late stage of the coating operation.

It has been the practice in coatin g fluorescent lamps of conventional contour to apply asuspension of luminescent material to the inner wall of an elongated glass tube by flushing the inner wall of the tube with the suspensionof luminescent material, permitting the excess to run olf and the residue to dry. One method of coating the elongated tube is to support the tube in a vertical plane and introduce the luminescent coating suspension at the top thereof and permit the suspension to drain down through thetube.

Another method of coating the elongated glass tube is to cause a column of luminescent material suspension to rise up into the tube from the bottom thereof until the entire tube is filled with portion of the lacquer in which the luminescent material is suspended must have a very high evaporation rate as compared to the evaporation rate of the solvent portion of the lacquer used in the coating of fluorescent lamps of the conventional type. I have also found that the viscosity of the fluorescent suspension used in coating curved tubes mu-st be substantially greater than the viscosity of the fluorescent suspension normally used in coating tubes for the conventional type of fluorescent lamps.

v The coating suspensions described in the following examples have proved satisfactory:

Example 1 Solvent composition: acetone," 25% ether Binder: 1200 sec. R. S. nitrocellulose wet with 30% ethanol Fluorescent material: 3500 white phosphor Plasticizer: 'diethyl phthalate equal to the weight of thewet nitrocellulose Viscosity: 4000 centipoises Specific gravity: 1.13

Rate of rotation: 2 hours per revolution (about .007 R. P. M.) l

ethyl Example 2 solvent composition:

50% carbon tetrachloride 45% normal pentane I 5% methanol Binder: centipoises N type ethyl cellulcs v Viscosity: 2000 centipoises Specific gravity: 1.48 Rate of rotation: 1%; hours perrevolution (about .014 R. P. M.)

Besides nitrocellulose, other suitable binders such as ethyl cellulose may also be used. The

only requirement of the binder is that it -be sol-uble in some mixture of solvent ,having a suffilciently fast evaporation rate, and that any organic portion of the binder be removable in a l subsequent baking operation.

Solvents that have been found to have suitable evaporation rates include such materials as.

acetone, ethyl ether, petroleum ether, methylene chl0ride, -chloroform, carbon tetrachloride, normal ,pentane, methyl acetate, and methanol.

Many others jhaving' evaporation rates similar to will be readily apparent .to those skilled in the art of lacquers and solvents.

Thespeed with which the suspension may be Q applied tothe curved glass tube is substantially slower than the speed with which the conventionaltubes may be .coated. If the tube is coated too fast astreaky uneven film results.

I have found that a satisfactory coating may be obtained by mounting the curved glass tube ,in' a'vertical plane partially filling it with the coating suspension and then rotating the tube slowly so as to deposit a thin even film of the suspension on the interior surface of the tube. As the tube isrotated a wet film of the coating suspension is deposited on the Wall of the tube and the coating suspension gradually moves around the rest of the tube. which has not as yetbeentouched by the coating suspension.

One of the primary. factors in determining the pension which has been deposited on the wall of The deposited fi1m.must be pergravity after the mainbody of the coating solurate of speed at which the tube may be rotated 3 is the speed with whichlthe .film of coating sustion hasbeen caused to move along within the I tube. This prevents streaks and light and heavy.

areas.

In order to coat that portion of thecurved tube which was not filled with the coating .sus-

pension initially, it is desirable to attach to the end of the tube adjacent the unfilled portion a suitable extension to collect the main body of 1 the coating solution as the-tube is rotated. Any convenient, arrangement may be made to accomplish this. For example, I have employed a glass tube extension coupled to the curved tube by means of a rubber tube coupling andla flask into which the coating-suspension drains.

l have found that 'a unifor-mQturning of the 1 tube at about 2 /2 hours ,per revolution is satisfactory to produce a uniformly smooth'coating when the suspension of Example 1 isemrotation rate of the order of 24/: hours per revolution has been found desirable when :theluminesseconds viscosity, and the solvent portion comcent material is suspended :in-a nitrocellulose lacquer, the nitrocellulose being of-the orderof 1200 "that the tube is being coated too fast when the main body of the coating suspension is caused to move through the tube at a speed which will leave a wet =fi lm of coating covering an area large enough to 'cause the excess coating which has not firmly adhered tothe wall of the tube or dried, to .run in a direction which is not substantially vertical. When this condition exists a streaky, uneven'coating results. Thus it may be seen that the speed with which a tube may be satisfactorily coated depends to a great extent on the speed With'which the deposited film of coating material dries. This in turn, is dependent in part on the evaporation rate of the solvent used in the nitrocellulose lacquer and on the rate of diffusion of the solvent vapor to'the open end of the tube.

In carrying out the process of my invention I have developed the equipment illustrated in the drawing. It will be apparent to those skilled in the art that other 'mechanicaldevices may be employed to practice the coating principles herein enunciated without departing from the spirit of their-ivention.

The apparatus-of my invention as shown in Figure 1, comprises a frame I provided with a plurality of sections 2 within which tubing holders 3 are disposed during the coating operation. When the tubing 'holders t are disposed in the several sections 2, they rest on two driven shafts 1 which are connected through a speed reduction mechanism 5 to a motor s which drives the shafts 4.

The tubing h01der 3 as shown in Figure 3 comprises a pair of spaced ring-s fl connected by the spacing members 8 and provided with a plurality of resilient clips '9 "which hold the tubing 10 in position. The auxiliary apparatus connected to the tubing 1-0 during the coating operationcomprises an extension "H connected to'the filling end of the tubing by means of a resilient coupling 12 and an extension Iii-connected t0 the discharge end of the tubing by means of a resilient coupling M, the extension l3 terminating in a reservoir [5. v

Figure 4 shows the tubing I0 with the extensionsH and f3 and the reservoir 15 connected thereto. This apparatus, hereinafter called the 1 tubing assembly fl, is-shown, in Figure4, in the position in which it is disposed at the start of the coating operation. A quantity of luminescent coa-ting suspension it is introduced into the tubing l Ii -through the extension I I. I have found it desirable to introduce the coatingsuspension l6 into thetubing I0 through the extension ll rather than introduce it directly into the tubing because it insures a smooth, even film of coating -on the wall of the tubing near the filling end. I have found that when an extension is not used and the coating suspension is introduced directly, the quality of the coating deposited adjacent this end of the tubing is not always entirely satisfactory. This'extens-ion may be removed after the coating suspension has been introduced into the tubing .19. a

The tubing assembly i l is mounted in the tubing holder 3 as shown in Figure-'3 and placed in one of the sections "2 in the frame I as shown in Figure 1. When the motor 6 is energized the driven shafts 4 turn and thereby cause the tubing holder 3 and the tubing assembly I! to rotate. The rotation of the tubing assembly I'I causes the column of coating suspension Hi to move through the tubing l8 depositing a film of coating in its wake.

It has been found desirable to cover the filling end of the tube 0 with a disc Or wafer after the extension II has been removed in order to control and limit the drying rate or the deposited film of coating suspension during the early stage of the coating operation. When this is not done it has been found that the film of coating deposited adjacent this end of the tubing dries faster than the film of coating deposited on the rest of the tube. A uniform drying rate is advantageous to insure a smooth even coating throughout the entire length of the tubing.

Figure 5 shows the disposition of the several elements of the tubing assembly I! after a large portion of the tubing has been coated. The numeral I8 designates the coating which has dried and the numeral I9 the coating which is still wet. The column of coating suspension l6 has moved through the tubing l so that now some of it has moved into and through the extension [3 and into the reservoir l5.

As was pointed out above, it is essential that the tubing be rotated at a speed which will insure the disposition of the drying area in a substantially vertical plane. This is illustrated in Figure wherein the area designated by. the numeral l9 represents the wet or drying coating. When a film of coating is deposited, some of the coating drys and adheres to the tubing and the excess of the suspension runs 011' under the influence of gravity. To insure a smooth even coating it is necessary that the drainage of the excess coating suspension be in substantially the same direction at all times. This is accomplished by utilizing a coating suspension of the type described above and rotating the tubing at a speed which will insure the disposition of the drying area in a substantially vertical plane at all times. This is illustrated in Figure 5 where the coated portion of the tubing [0 which has not yet dried and which is represented by the numeral I9 is disposed in a substantially vertical plane.

What I claim is:

The method of coating a curved glass tube with a suspension of luminescent material which comprises: disposing the tube substantially vertically; introducing a body of said suspension into said tube through one end thereof; rotating said tube to thereby effect movement of the body of said suspension therein and deposition of a coating of said suspension on the wall thereof; and drying said deposited coating, the speed of rotation of said tube being such that that portion of the tube on which the coating which has not yet dried has been deposited is disposed substantially vertically.

ERIC L. MAGER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 949,156 Murray Feb. 15, 1910 1,188,776 Hewitt June 27, 1916 1,956,722 Kennedy May 1, 1934 2,179,288 Freck Nov. 7, 1939 2,192,869 Pearce Mar. 5, 1940 2,318,060 Cortese May 4, 1943 2,413,437 Demb Dec. 31, 1946 FOREIGN PATENTS Number Country Date 437,755 Great Britain Nov. 5, 1935 

